The use of electrostatic powder coating techniques to paint electrically conductive substrates, such as metals, is well known and successfully employed. Using this method, a powder coating material is statically charged or ionized to a positive polarity or negative polarity, and then sprayed or blown onto a grounded, conductive article to which it adheres. The electrostatic attraction between the paint and the grounded article results in a more efficient painting process with less wasted material, and a thicker, more consistent paint coverage, particularly on articles that have a complex shape. Once coated, the article is then baked. In electrostatic painting, a powder coating material is statically charged and applied using standard powder coating equipment. With electrically conductive substrates, a static electric potential is generated between the paint and the substrate to be painted resulting in an attraction of the paint to the object.
When articles fabricated from metals are painted, the metal, which is inherently conductive, is easily grounded and efficiently painted.
However, in recent years, there has been an emphasis on the use of polymeric materials in the manufacture of articles, particularly in applications requiring reductions in weight and improved corrosion resistance, such as automotive applications. However, polymers typically used in such processes are insufficiently conductive to efficiently obtain satisfactory paint thickness and coverage when the article is electrostatically painted.
On poor electrical conductors such as polymeric materials, the conventional electrostatic coating techniques are not as successful because an electric charge potential must exist between both the substrate and the paint. If an object has poor electrical conductivity, it cannot be efficiently electrostatically charged and cannot, therefore, be efficiently electrostatically painted. Furthermore, on non-conductive surfaces, low humidity levels can have a negative impact on the quality of the bond of the powder coating to the surface.
Even so, electrostatic painting techniques are still desirable for use due to the benefits, especially for large scale commercial operations, including less loss of paint than with the use of other painting techniques such as spraying a liquid paint, and the quality of the coating is quite good because the method allows for a uniform distribution of paint without the entire surface being easily accessible. Materials having little or no conductivity such as plastics, may first be coated with a conductive primer or “prep” coating, and then electrostatically painted.
Some specific examples of methods of applying an electrostatic charge to surface having little or no conductivity include the addition of conductive fillers to polymers, for instance, application of a conductive primer such as a quaternary amine, However, the conductivity from these treatments, as well as the physical and/or surface characteristics may be less than desirable for certain applications.
The use of conductive primer compositions to prime the article in order to increase its conductivity is also known. However, depending on the particular primer employed, the cured primer may have adhesion, surface smoothness, hydrolytic stability, and durability characteristics, which are less than desirable for a particular application. Additionally, such primers compositions may contain volatile organic solvents, the emission of which during the priming process may be undesirable, as well as environmentally unfriendly. Further, each of the treatments described above can be expensive.